Genetic counselling and/or testing to identify mutation status and risk of colorectal cancer (CRC) is not utilised in approximately half of people from mutation-carrying families. We studied perceived CRC risk and reasons for declining in 26 participants (mean age 43.1 years, 14 women) in the Australasian Colorectal Cancer Family Registry. All were relatives of mismatch repair gene mutation carriers, who had not been diagnosed with any cancer and had declined an invitation to attend genetic counselling and/or testing. Bounded estimates of perceived CRC risk over the next 10 years, understanding of genetic testing and CRC risk, reasons for declining testing and self-reported colonoscopy screening were elicited during a face-to-face semi-structured interview. A sub-group of decliners (31%) unconditionally rejected genetic testing compared to conditional decliners who would consider genetic testing in the future. Mean perceived 10-year risk of CRC was 54% [95% CI 37, 71] in unconditional decliners, compared with 20% [95% CI 5,36] in people who conditionally decline genetic testing, after adjusting for potential confounding factors (age, gender and reported screening colonoscopy). The unconditional decliner group perceive themselves to be at 3.26 times higher risk than conditional decliners. General practice interventions may increase testing uptake and/or screening for high-risk under-serviced individuals, whose health behaviours are inconsistent with their risk perceptions.